1. Field of the Invention
The invention relates to an array substrate for use in such as a liquid crystal display, and a method of manufacturing the same.
2. Description of the Related Art
Any liquid crystal display of active matrix type comprises a pixel substrate, a countersubstrate, and a liquid crystal layer interposed between the pixel substrate and the countersubstrate. Of these components, the pixel substrate is a transparent substrate made of, for example, glass. On the transparent substrate, a plurality of gate electrode lines are formed, and a plurality of drain electrode lines are formed intersecting the gate electrode lines. Further, thin-film transistors (TFTs), each made of amorphous silicon or the like, are arranged near the intersections of the gate electrode lines and drain electrode lines. Each TFT is connected not only to a gate electrode line and a drain gate line, but also to a pixel electrode that is a transparent electrode made of, for example, indium-tin oxide (ITO). The countersubstrate is a transparent substrate made of, for example, glass. On the countersubstrate, a counterelectrode is made of transparent material such as ITO.
In recent years, various methods have been proposed, which are designed to manufacturer an array substrate of so-called “top ITO structure.” In each of these methods, pixel electrodes are formed on the uppermost layer of the pixel substrate in order to reduce the number of steps of manufacturing the pixel substrate, as described in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2005-242372. To ensure electrical connection of the lines running on the layers of the top ITO structure, the layers have contact holes. Through the contact holes, the lines extend through layers, whereby any line provided on one layer can be electrically connected to a line or lines provided on any other layer.
In such a method of manufacturing an array substrate, the gate electrode lines and drain electrode lines can be formed at the same time the TFTs for pixels are formed. This can serve to reduce the manufacturing steps.
It is desired that the contact holes should be gently tapered from an upper layer toward a lower layer. If each contact hole is so tapered, a line can extend through the contact hole, not cut at all. As a result, the line reliably connects two associate lines provided on the upper and lower layer, respectively.
A plurality of contact holes of different depths may be made in the same manufacturing step. In this case, a shallow contact hole is exposed to etchant for a longer time than a deep contact hole. The shallow contact hole will be over-etched. If over-etched, the contact hole may be tapered in the reverse direction, that is, from a lower layer to an upper layer. (This result will be called “reverse taper.”)
The line formed in any contact hole of reverse taper may be cut at any part. If this happens, the associated two lines provided on the upper and lower layers, respectively, may not be connected.